Semiconductor light emitters are well known in the art. Semiconductor light emitters include light emitting diodes (LEDs) and laser diodes. LEDs are devices of choice for many display applications because of the advantages LEDs have over other light sources. These advantages include a relatively small size, a low operating current, a naturally colored light, a low power consumption, a long life, a maintained high efficiency (power in versus light output), an acceptable dispersal of light, and a relatively low cost of manufacture compared to other light sources.
Applications for LEDs include the replacement of light sources, such as incandescent lamps, especially where a colored light source is desired. LEDs are often used as display lights, warning lights and indicating lights. LEDs are advantageous in such applications, because they emit light in a relatively narrow range of wavelengths corresponding to a particular color. LEDs have not conventionally been used for lighting applications where a bright white light is needed, due to their inherent color.
Recently, blue emitting LEDs have been used to provide a whitish light, for example as backlights in liquid crystal displays and as replacements for small conventional lamps and fluorescent lamps. As discussed in chapter 10.4 of "The Blue Laser Diode" by S. Nakamura et al., pages 216-221 (Springer 1997), incorporated herein by reference, white light LEDs can be fabricated by forming a ceramic phosphor layer on the output surface of a blue emitting semiconductor LED. The blue LED is an InGaN single quantum well LED, and the phosphor is a cerium doped yttrium aluminum garnet Y.sub.3 Al.sub.5 O.sub.2 :Ce.sup.3+ ("YAG:Ce"). The blue light emitted by the LED excites the phosphor, causing it to emit yellow light. The blue light emitted by the LED is transmitted through the phosphor and is mixed with the yellow light emitted by the phosphor. The viewer perceives the mixture of blue and yellow light as white light.
However, the blue LED--YAG:Ce phosphor white light illumination system suffers from the following disadvantages. The blue LED--YAG:Ce phosphor system produces white light with a high color temperature ranging from 6000 K to 8000 K, which is comparable to sunlight, and a typical color rendering index (CRI) of about 70 to 75. While the blue LED--YAG:Ce phosphor illumination system with a relatively high color temperature and a relatively low CRI is acceptable to customers in the far east lighting markets, the customers in the North American markets generally prefer an illumination system with a lower color temperature, while the customers European markets generally prefer an illumination system with a high CRI. For example, North American customers generally prefer systems with color temperatures between 3000 K and 4100 K, while European customers generally prefer systems with a CRI above 90.
The color temperature of a light source refers to the temperature of a blackbody source having the closest color match to the light source in question. Generally, as the color temperature increases, the light becomes more blue. As the color temperature decreases, the light appears more red. The color rendering index (CRI) is a measure of the degree of distortion in the apparent colors of a set of standard pigments when measured with the light source in question as opposed to a standard light source. Light sources having a relatively continuous output spectrum, such as incandescent lamps, typically have a high CRI, e.g. equal to or near 100. Light sources having a multi-line output spectrum, such as high pressure discharge lamps, typically have a CRI ranging from about 50 to 80.
Another disadvantage of the above LED lamp relates to the efficiency of the lamp. The lumens per watt (LPW) of the above-described LED is in a range of about 5 LPW to about 10 LPW. LED radiation at about 5 LPW to about 10 LPW with a CRI of about 70-75 may not be acceptable for many lighting applications. For example, many lighting applications require a LPW that is 15 LPW or more, with a CRI maintained at or above 85. Known LED light sources do not provide a single LED with a sufficient LPW and CRI for most generalized lighting applications, especially for white light.
Therefore, a semiconductor light source that produces a bright white light is needed. Also, efficient green light emitting phosphors are needed for phosphor conversion material blends to produce a bright white light.